(1) Field of the Invention
The invention relates to nuclear reactors, and, particularly, to a system for shutdown of the reactor, including a valve assembly to abate "chattering" during operation.
(2) Brief Description of the Prior Art
A nuclear reactor converts nuclear energy, i.e. atomic energy, into thermal energy. The nuclei of atoms consist, broadly speaking, of the elementary particles called protons and neutrons. The protons have a positive electric charge, whereas neutrons have no charge and are electrically neutral. Very powerful forces of action act between these particles, collectively referred to as "nucleons" and hold them together in the nucleus.
Heavy atomic nuclei are, however, not so stable as light ones, because in the former the repulsive forces exerted by the protons loosen the structure of the nucleus. For this reason it is possible to cause fission of heavy nuclei, such as those of uranium 235, by bombarding them with free neutrons. As a result of this impact, nuclei of uranium 235 are set vibrating, and these vibrations become so violent that the nucleus is split up into several parts, i.e., into a barium and a krypton nucleus. The "fission products" travel at considerable velocity, collide with matter somewhere in the reactor, and give off their kinetic energy as heat. This is a conversion of nuclear energy into heat.
In addition to the fission products and heat formed in the fission of uranium, however, new neutrons are also released, which, in turn, can cause the fission of more uranium atoms. In this way a chain reaction is initiated. The neutron strikes the uranium 235 nucleus and briefly forms the intermediate product, uranium 236, which disintegrates spontaneously into strontium and xenon. Additional neutrons are released in this fission process which sustain the chain reaction.
In order to be able to utilize these neutrons, which are emitted from the parent nucleus at high velocity, for further fissile processes, they have to be slowed down. Low-velocity neutrons are much better suited to split atoms than high-velocity neutrons. The slower neutrons can interact with the uranium nucleus for a greater length of time, whereas faster neutrons are in the vicinity of nucleus for too short a time to initiate the fission process. The velocity of the neutrons is moderated by causing them to collide with light atoms, large numbers of which must be incorporated in a reactor for this purpose. Materials consisting of such lighter items are, for example, graphite and water. The neutrons which have been slowed down in this way will then cause fission of further uranium 235 nuclei. Each fission process gives birth to fresh neutrons, so that the chain reaction is self-sustaining and the reactor is consequently kept in operation.
In the case of a water-moderated reactor, uranium is installed in the form of metallic rods in a vessel filled with water. Fission takes place within the uranium and neutrons which are released in the process and travel into the surrounding water, where they collide with the light hydrogen and oxygen atoms and are moderated, i.e., they lose velocity. These slowed-down neutrons re-enter the uranium rods with a certain probability and there they cause other fission reactions to take place. The fission products formed as a result of these reactions give off their energy to the uranium, which, in turn, transmits it to the water. This hot water is used to turn a turbine which generates electricity.
To ensure that the reactor will not stop functioning nor become excessively overheated, the rate of neutron formation inside it has to be controlled with considerable accuracy. This is done by means of the control rods, which consist of a neutron-absorbing material and which are inserted into the reactor core to an accurately variable depth. This depth of penetration must be just enough to ensure that, on an average, per fission only one neutron remains available to produce another fission reaction. Since the fission products are highly radioactive, the reactor must be enclosed in a thick casing of concrete called the "shield".
A type of water-moderated reactor is sometimes referred to as a boiling water reactor. In these reactors, an emergency system is provided to shut down the reaction in the event of an indication that control of the reaction may be lost or difficult. Typically, such systems incorporate a hydraulic control line having a control fluid therein, such as charging water, or the like, which is pressured by means of an accumulator tank having a piston having one side exposed to a gaseous substance, such as nitrogen. This control line extends to one face of a main drive piston. A charging water header or conduit supplies control fluid into the control line and the accumulator. A check valve assembly is disposed between the charging water header and the accumulator assembly to selectively permit charging and re-charging of the control line and the accumulator.
In the event that it is necessary to shut down the nuclear reactor vessel, a solenoid actuated valve on the control line between the accumulator and the main drive piston is opened to permit fluid, pressured on the order of 1,800 p.s.i.g., to drive the main piston interiorly within the reactor vessel. As the main drive piston is moved, pressure in the control line decreases to approximately 1,100 p.s.i.g. and the check valve assembly downstream of the charging water header immediately opens to recharge the control line and the accumulator.
The flow of fluid in the control line during shutdown is extremely fast and the re-charging of the accumulator is initiated almost spontaneously. It has been found that such recharging results in the ball element of the check valve assembly downstream of the charging water header and upstream of the accumulator to bouncing up and down violently, or "chattering" causing, in turn, large massive hydrodynamic loads to take place on the piping system for the shutdown assembly.
The present invention is directed to abating this problem.